Inclusion-Enhanced Ceramic for Improved Thermal Shock Tolerance

As discussed in the announcement document, the apertures of high-speed vehicles may be subjected to aggressive environmental conditions including rain impact, high temperatures, and highly transient aerothermal heating loads, all of which may adversely impact sensing capabilities or the structural survivability of the aperture material. The large thermal gradients caused by highly transient thermal loads are of particular concern due to the significant bending stresses they can induce (compressive near the hot surface, tensile near the cool surface). These stresses can be particularly problematic for the ceramic materials typically used for infrared (IR) apertures, which are often brittle in nature with low failure strains. The aperture is said to fail in thermal shock when stresses of this general type exceed the structural capability of the material. The proposed Phase II program aims to build upon a material that, to the knowledge of the proposers, is among the most recent materials to demonstrate high optical transmittance in MWIR, as found both in the literature and in ongoing work by the fabrication partner. While the material has not yet been tested in thermal shock conditions, the mechanical properties measured to date indicate it has the potential to be a high performer, and it offers several other advantages over incumbent materials. The primary potential disadvantage of the material is its relatively high CTE. In severe thermal shock conditions, this characteristic could cause excessive stresses in the aperture, the frame, or the interfacing portion of the aeroshell, potentially resulting in structural damage or failure of any or all of these components. The high CTE could also cause the aperture to deform to a level that causes distortion of the imagery seen by the seeker. The proposed effort therefore aims to develop an enhanced version of the material which incorporates inclusions at an appropriate through-thickness location to reduce the stresses and deformations associated with thermal shock (thereby increasing its thermal shock tolerance) while maintaining or improving upon the optical performance of the base material. To achieve this goal, the Phase II Base Period includes: vehicle-level simulations to define relevant thermal shock conditions; simultaneous optical and thermo-structural material modeling to reach an optimized material design; fabrication and testing of test coupons for optical and mechanical properties; and fabrication, testing, and analysis of specimens under thermal shock conditions in LHMEL. The Option Period would address: manufacturing scale-up; fabrication of a larger-scale item; detailed design of the complete aperture, frame, and interfacing aeroshell region; and the simulation of aero-optical performance of an aperture that utilizes the enhanced material architecture.